In paper manufacture, there are various steps which require the treatment of aqueous solutions for example to remove or align fibers in the solution. This treatment is generally carried out by means of slotted hole screens in which wire-shaped or bar-shaped longitudinal members that define the screen slots are mounted on cross bars. Slotted hole screens in general also include so called slit screens that have shorter slot lengths or discontinuous slots. The screens are used in different forms, for example as cylindrical screen baskets, flat screens, curved screens, half shell screens, drum screens or the like.
Such screens are used in sorters, reject sorters and so-called fractionators. The sorters serve to remove impurity materials, such as contaminant particles, from aqueous fiber suspensions which, in this phase of treatment, have an approximately 0.5 to 4% fiber content and about 96 to 99.5% water content, while the fractionators serve to separate two fractions having fibers of different lengths from one another and to hold back the longer fibers at the screen and remove them separately. The screens are also used in reject sorters and drum screens which operate on a material consistency of up to 20%.
In known manufacturing processes of such slotted hole screens, the longitudinal members and the cross bars are joined by means of welding or by wrapping them around each other. A common welding process which is used is the known so called resistance welding in which an electrical current creates a heat of fusion at the contact points between the longitudinal members and the cross bars to physically join the two pieces. In principle, however, other welding processes are also suitable. The welding process has the disadvantage that the screens made in this manner have undesirable burr formations as well as an undesirable roughness, recesses, gaps or bumps in the welded joints.
In the case of wrapped joints in which the longitudinal members are wrapped around the cross bars, undesirable recesses and edges are formed in the joints. During use of these screens, fibers from the fiber suspension can be caught in the joints, which results in the danger that further fibers attach themselves to the caught fibers and form cohesive flocks, webs or plaits. When flocks, webs or plaits formed in this way release themselves from the screen after a certain period of operation of the slotted hole screen and these are flushed into the flow of material behind the screen, the desired sorting effect or fractionating effect of the screen is no longer assured. This is extremely disadvantageous in paper manufacture, in which it is important that the paper fibers in the fiber suspension are separated to the maximum extent before they are applied onto the water extracting screen, and it is also important that no coherent flocks or webs are formed which considerably worsen the quality of the paper to be produced and can lead to interruptions in production. Blockages of the sorting screen can also arise.
It is, therefore, an object of the invention to provide a process for manufacturing slotted hole screens in which the screen joints are formed in such in such a manner that the danger of the catching paper fibers, and thus of forming flocks and/or webs of fibers as well as the blocking of the screen is entirely or substantially reduced.
It is another object of the invention to provide a process for manufacturing slotted hole screens in which the strength of the joint locations is increased.